1. Field of the Invention
This invention pertains to a velocity change sensor or accelerometer used in motor vehicles for detecting sudden changes in velocity and for activating in response a passenger restraining device such as an air bag. More particularly, this device employs an element which is biased in a first position by a magnet and moves to a second position in response to a sudden deceleration to establish an electric contact. An element is provided for shaping and concentrating the magnetic field generated by the magnet.
2. Background of the Invention
Studies have been made which indicate that injuries in motor vehicle accidents, especially at high speeds, can be substantially reduced or eliminated by the use of passenger restraint systems. These systems include an inflatable balloon usually termed an air bag which normally is stored away in the instrument panel or the steering wheel. When the motor vehicle is subjected to a sudden deceleration, the air bag is inflated and is deployed automatically in a position which cushions the passengers, restrains their movement and prevents contact between them and the automobile interior such as the windshield, the steering wheel. the instrument panel and so on. Of course, a crucial element of all such systems is the velocity change sensor or accelerometer which initiates the inflation and deployment of the air bags. The motion of the motor vehicle must be carefully and precisely monitored so that the air bags can be deployed very fast, before the passengers suffer any substantial injury. A velocity change sensor is disclosed in U.S. Pat. No. 4,329,549 assigned to the same company as the present invention. This sensor comprises a tubular housing surrounding a magnetically permeable metallic shell, a metal ball and a magnet biasing the ball toward a first end of the shell. At the second end of the shell there are a pair of electrical contact blades. The sensor is positioned in the motor vehicle in an orientation such that when the motor vehicle experiences a deceleration which exceeds a preset level, the ball moves from the first toward the second end, making contact with the two blades. Because the blades and the ball are made of on electrically conducting material, when the ball contacts the blades, an electrical path is established between the two blades. This electrical path is used to initiate the deployment of the air bags. One problem with this type of device involves the field generated by the magnet. If the sensor is disposed in a metal enclosure, the sidewalls of the enclosure usually concentrate and shape the field so that it is bottled in a region close to the metal sleeve. However if an enclosure which has no effect on a magnetic field, such as one having a low magnetic permittivity, such as a non-metallic enclosure, the field generally diverges away from the housing. Therefore the intensity of the magnetic field at the sleeve is lower when the housing the sensor is disposed in a non-metallic enclosure than in a metallic enclosure. A lower magnetic field degrades the performance of the sensor. Furthermore, it has been found that very sensitive and discriminative acceleration sensor systems may be made by mounting two sensors in a common housing. However, if these sensors are two close together, the fields from the two magnets interfere and produce undesirable responses.